1. Field of the Invention
The invention is directed to new desensitizers for hypersensitive teeth and to methods of making and using such desensitizers.
2. Description of Related Art
Dentinal hypersensitivity causes pain in the mouth of a patient when a nerve in an affected tooth is exposed to certain external stimuli, including temperature and tactile stimuli. One possible source of dental hypersensitivity is that the dentin of affected teeth is over-exposed to the stimuli due to injury, disease or some other reason. Dentin generally contains channels, called tubules, that allow material and energy transport between the exterior of the dentin and the interior of the tooth where the nerve resides. Exposure of these tubules to external stimuli can cause irritation of the nerve in a tooth, leading to discomfort. Although the exact mechanism of hypersensitivity remains under investigation, recent investigations have shown that the pain triggered by air currents is related to the number of exposed tubules per unit area of dentin (Kontturi-Narhi, Dentin Hypersensitivity--Factors Related to the Occurrence of Pain Symptoms. Kuopio University Publications B. Dental Sciences 5.) According to the hydrodynamic theory of dentin sensitivity mechanical and thermal stimuli of the exposed smear layer free dentin surface, induces minute movements of the intratubular fluid. These fluid movements induce pain encoding nerve responses in the intradental nerves located near the dentin/pulp border. Recent investigations have strengthened the experimental evidence in support of this relationship (B. Matthews and N. Vongsavan Archs Oral Biol, 39 (Suppl): 875-955, 1994).
Dental hypersensitivity is generally treated by either treating the nerve in the tooth to make it less sensitive to stimuli or by blocking or occluding the tubules to prevent or limit exposure of the nerve to external stimuli and limit the stimulus triggered fluid movements in the dentinal tubules.
Treatments that directly affect the nerve generally interfere with the electrolyte balance near the nerve to affect the outer membranes of the nerve so that the nerve does not "fire" as frequently or as strongly as an untreated nerve. Useful agents in treating dental hypersensitivity in this manner include potassium nitrate, as set forth in U.S. Pat. No. 3,863,006 to Hodosh, issued Jan. 28, 1975, potassium chloride, as set forth in U.S. Pat. No. 4,751,072 to Kim, issued Jun. 14, 1988, potassium bicarbonate, as set forth in U.S. Pat. No. 4,631,185 to Kim, issued Dec. 23, 1986, and strontium chloride, as set forth in U.S. Pat. No. 3,122,483 to Rosenthal, issued Feb. 25, 1964.
Occlusion of the tubules provides an alternative method of treatment. Useful reported agents include polymeric agents such as Carbopol, as set forth in U.S. Pat. No. 5,270,031 to Lim et al., issued Dec. 14, 1993, and certain polystyrene beads, as set fort in U.S. Pat. No. 5,211,939 to Turesky et al, issued May. 18, 1993.
Apatite can also be an anti-hypersensitivity agent. U.S. Pat. No. 4,634,589 to Scheller, issued Jan. 6, 1987, and U.S. Pat. No. 4,710,372, issued Dec. 1, 1987, also to Scheller, disclose dentifrices for hypersensitive teeth containing apatite having an average particle size of less than 10 microns and optionally a local anesthetic. No other soluble mineral salts are permitted to exert any interfering effect in these patents. The apatite reduces the diameter of the dentin channels.
Montmorrolinite clay has also been reported as a desensitizing agent in U.S. Pat. No. 4,992,258 to Mason, issued Feb. 12, 1991. Unfortunately, montmorrolinite clay is not compatible with most known fluoridating agents and thus has limited use. In addition, montmorrolinite clay loses its ability to thicken a dentifrice and has reduced ability to block tubules in the presence of inorganic salts, such as potassium salts, so its use as a desensitizer is limited.
Other types of clays have been used in dental applications, although not in a desensitizing capacity. With the advent of clear gel dentifrices, hectodte clays, especially laponite clays, have been used as thickeners for dentifrices, for example as reported in U.S. Pat. No. 4,069,310 to Harrison and in Mayes, B., "Synthetic Hectoritc--A New Toothpaste Binder," International Journal of Cosmetic Science, 1,329-340 (1979). While thickeners and binders are usually found in dentifrices at about 1% by weight, the Harrison patent indicates that the thickener may be present in amounts up to 5% by weight. Indeed, the Mason patent discussed above indicates that laponite may be one of a number of thickeners used in the dentifrice, despite its teaching of montmorrolinite clay as a desensitizer.
U.S. Pat. No. 4,474,750, Gaffar et al., issued Oct. 2, 1984, discloses toothpaste, cream or gel in which the thickening agent can be Laponite CP or SP in an amount up to about 10% by weight. There is no disclosure in the patent that the Laponite is incorporated in an oral composition for the purpose of treating hypersensitive teeth.
U.S. Pat. No. 4,081,526 to Asakawa et at., issued Mar. 28, 1978, discloses dentifrice compositions comprising 0.5 to 13% of a hectoritc clay such as Laponite, for removing plaque from the teeth.
Despite the ongoing work in the field of desensitizers, a strong and long-felt need remains in the art for an effective tubule blocking agent that is compatible with fluorides and other conventional dentifrice ingredients. This agent must work well yet not be distasteful to use. It must be stable for the typical shelf life of a dentifrice, and it should be affordable.